System and method for searching a database or data sharing system for the presence of data

ABSTRACT

A database search system ensures the privacy of a search request and the security of a database with high processing efficiency. The system comprises a database server residing on a network that stores predetermined data, and a client terminal issuing to the database server a search request for inquiring for desired data while keeping the desired data secret. The database server further comprises: a database storing predetermined data; and a search response message generation unit for receiving through a network the data search request in which data to be searched for is kept secret, searching the database by a predetermined computation based on the search request and a list of the data stored in the database while a data item involved with the search request is kept secret, and generating a response to the search request.

CROSS-REFERENCE TO RELATED APPLICATIONS

Under 35 U.S.C. 120, this application is a Continuation Application andclaims priority to U.S. application Ser. No. 10/421,256, filed Apr. 22,2003, entitled “METHOD FOR SEARCHING A DATABASE OR DATA SHARING SYSTEMFOR THE PRESENCE OF DATA,” which claims the priority of Japanese patentapplication, Serial No. JP2002-176339, filed Jun. 17, 2002, entitled“DATABASE SEARCH SYSTEM, DATA SHARING SYSTEM, AND DATA SEARCH METHOD FORTHEM,” all of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention generally relates to a database search system, andmore particularly to a system for performing a search to determinewhether a certain database stores particular data of interest.

BACKGROUND OF THE INVENTION

One of the services provided on a network is a data search service fordata stored in a database. A client who wants to use the database sendsa search request (query) to a database server on the network to inquirewhether a desired data is in the database and receives a search resultin response to the query.

In such use of the database, a search may be desired only to determinewhether the database stores a particular data or not. In this case, thedatabase server replies to the search request by returning informationon the presence or absence of the requested data, rather than returningthe data itself. In an exemplary situation, a user may wish to determinewhich technical database stores particular technical information among aplurality of technical databases. If any technical database is found tostore the desired data, another request may be made for transmission ofthe data from that technical database storing the data.

This search approach poses some concerns: the privacy of the searchrequest of the client at the system sending the search request, and thesecurity of the database.

The user may not wish the details of the search request available tounauthorized persons. The search request contains information on theuser's interest. For example, a company may search a technical databaseof a competitor. Revealing the details of that search to the competitor,such as the type of technique the company is researching, can pose asignificant problem, possibly exposing the company's business strategy.On the other hand, an open database shared among companies of the sametrade leads to a vigorous industry. Therefore, it would be extremelyuseful if a database of the competitor can be searched while the contentof a search request is kept secret.

In addition, the owner of the database may wish to maintain data anddatabase security. For example, some systems charge for data retrievalfrom a database. In such systems, charging for the data would bemeaningless if unlimited amounts of data could be retrieved from thedatabase in one search. Even in charge-free systems, it is desirable toavoid providing a user with excess search results, because the databaseitself contains information on interests or strategies of the databaseowner.

Several protocols have been proposed to address the issues of databaseaccess and security.

To perform a database search while hiding a list L_(i) of data stored ina database and the value α of a data number to be searched for, onemethod uses a one-way hash function H. FIG. 8 describes thisconventional method. Suppose that a database server S_(i) hides a listL_(i)={x_(i, 0), . . . , x_(i, n-1)} from a client A, and the client Ahides α. The database server S_(i) then provides L′_(i)={H(x_(i, 0)), .. . , H(x_(i, n-1))} to the client A, and the client A checks whetherany one matches H(α), where H(•) is a one-way hash function. In thiscase, neither the client A nor the database server S_(i) needs todisclose the data numbers.

Another method as described by Naor, M. and Pinkas, B., “ObliviousTransfer and Polynomial Evaluation,” Proc. of STOC '99, uses ObliviousPolynomial Evaluation (abbreviated as OPE hereafter). This is a protocolcharacterized in that only a result of substituting an input into afunction is received while the input is hidden.

More specifically, this method is characterized as follows. A recipientA has a secret value α, and a sender B has a secret polynomial f(x). Therecipient A can know a polynomial f(α) by the OPE protocol, but obtainno information on f(β) for inputs β(≠α) other than α. On the other hand,the sender B cannot know α even after execution of the OPE protocol.

FIG. 9 describes a conventional method using OPE. First, a client A anda database server S_(i) choose random n-order polynomials f_(A)(x) andf_(Si)(x), respectively. The client A obtains f_(Si)(α) from thedatabase server S_(i) by OPE. The database server S_(i) obtainsf_(A)(x_(i, 0)), . . . , f_(A)(x_(i, n-1)) by OPE. The database serverS_(i) sends L′_(i)={f_(Si)(x_(i, 0))+f_(A)(x_(i, 0)), . . . ,f_(Si)(x_(i, n-1))+f_(A)(x_(i, n-1))} to the client A. The client A canknow whether α∈L_(i) by computing the following equation:

ã=f _(Si)(a)+f _(A)(a)

and verifying the following equation:According to this method, no excess information about α∈ L_(i) isexposed.

A protocol for a special form of OPE based on the DecisionalDiffie-Hellman (DDH) problem is presented in Lindell, Y. and Pinkas, B.,“Privacy Preserving Data Mining,” Proc. of CRYPTO2000, LNCS.

The DDH problem is a decisional problem of distinguishing between D=<g,h, g^(a), h^(a)> (this form is called a “Diffie-Hellman tuple”) andR=<g, h, g^(a), h^(b)> with significant probability on GF(q), where g,h∈ GF(q) and a, b∈ Zq are randomly chosen. Algorithms for solving thisproblem in polynomial time in the size of q are not known. Therefore, itis often used as means of ensuring the security of cryptography. Themethod using OPE in this approach is different from the method describedpreviously in that B having the polynomial f(x) returns g^(f(x)) (notf(x) itself) for the input α from the client A.

Thus, search methods have been proposed in which the privacy of a searchrequest and the security of a database are taken into consideration whena database search is requested. However, these methods present numerousdisadvantages.

The conventional method using a one-way hash function allows the clientA to search for unlimited different items. For example, if the client Awants to know whether α′(≠α) is included in the list L_(i), the client Amay simply check whether H(α′) ∈ L′_(i). By repeating this, the client Amay ultimately determine the content of the list L_(i). That is, thismethod presents a problem with achieving secrecy (security) of thedatabase.

Another conventional method using OPE requires communication in four ormore passes to ensure the privacy of the search request and the securityof the database, as shown in FIG. 9. Therefore, the method presentsproblems relating to high communication cost and complicated processing.

Thus, there is a need for a database search system and method thatensures the privacy of a search request and the security of a databasewith high processing efficiency. The need for such a system hasheretofore remained unsatisfied.

SUMMARY OF THE INVENTION

The present invention satisfies this need, and presents a system, acomputer program product, and an associated method (collectivelyreferred to herein as “the system” or “the present system”) forproviding a database search system that ensures the privacy of a searchrequest and the security of a database with high processing efficiency.To achieve the above object, the present system is provided as adatabase search system comprising: a database server residing on anetwork and storing predetermined data; and a client terminal issuing tothe database server a search request for inquiring for desired datawhile keeping the desired data secret.

The database server comprises a database storing predetermined data anda search response message generation unit. The search response messagegeneration unit receives through a network a data search request inwhich the requested data is kept secret, searches the database by apredetermined computation based on the search request and a list of thedata stored in the database while a data item involved with the searchrequest is kept secret, and generates a response to the search request.

Specifically, the search response message generation unit of thedatabase server receives a search request for predetermined data througha network, generates a polynomial having a data item involved with thesearch request as a root based on the search request that contains aconfidentiality request for the query and the content of the retrieveddata, and a list of the data stored in the database, solves anevaluation problem in which a result of substitution of the data iteminto the polynomial becomes zero by Oblivious Polynomial Evaluation, andgenerates a response to the search request.

In one embodiment of the present system, the search response messagegeneration unit of the database server randomly generates the polynomialusing a random number.

The client terminal comprises a search request message generation unitand a search result determination unit. The search request messagegeneration unit encrypts a data item involved with desired data andgenerates the search request for inquiring of the database server forthe desired data. The search result determination unit for receives anevaluation result for the search request obtained by ObliviousPolynomial Evaluation from the database server and obtains informationindicating whether the database server has the desired data based on theevaluation result.

The present system is also implemented as a data sharing system forsharing data in a peer-to-peer network system. The data sharing systemcomprises: a first peer issuing to other peers a search request forinquiring for desired data while keeping the desired data secret; and asecond peer searching its database and sending an obtained search resultto the first peer. The database search performed by the second peer usesa predetermined computation based on the search request received fromthe first peer and a list of data stored in the database; the desireddata involved in the search request is kept secret.

Any peer in the peer-to-peer network may be the first peer describedabove. Each of other peers (usually all other peers unless otherwiselimited by settings) may then be the second peer described above.

More specifically, the second peer generates a polynomial having a dataitem involved with the search request as a root based on the searchrequest received from the first peer and a list of data stored in itsdatabase. The second peer then solves an evaluation problem in which aresult of substitution of the data item into the polynomial becomeszero, and sends an obtained evaluation result to the first peer.

The present system is also implemented as a data search method forsearching a database storing predetermined data to determine whetherdesired data is stored in the database. The data search methodimplements the following steps: generating a search request forinquiring of the database for the desired data; generating a polynomialhaving a data item involved with the search request as a root based onthe search request and a list of the data stored in the database andsolving an evaluation problem in which a result of substitution of thedata item into the polynomial becomes zero; and obtaining informationindicating whether the database stores the desired data based on anevaluation result.

Another data search method of the present system implements thefollowing steps: encrypting a data item involved with desired data usinga first key and generating a search request for inquiring of thedatabase for the desired data; performing a database search by apredetermined computation based on the search request and a list of thedata stored in the database while the desired data involved in thesearch request is kept secret; and based on a result of the databasesearch, obtaining information indicating whether the database stores thedesired data by a predetermined computation using a second keycorresponding to the first key.

The present system may also be implemented as a program for controllinga computer so that the computer functions as the above describeddatabase server or client terminal (terminal device). The program may bestored in and distributed as a magnetic disk, optical disk,semiconductor memory, or other storage media, or distributed through anetwork.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features of the present invention and the manner ofattaining them will be described in greater detail with reference to thefollowing description, claims, and drawings, wherein reference numeralsare reused, where appropriate, to indicate a correspondence between thereferenced items, and wherein:

FIG. 1 is a schematic illustration of an exemplary operating environmentin which a database search system of the present invention can be used;

FIG. 2 is a schematic illustration of the high-level architecture of thedatabase server and a client terminal utilizing the database searchsystem of FIG. 1;

FIG. 3 is a block diagram illustrating the concept of a list searchaccording to the database search system of FIGS. 1 and 2;

FIG. 4 is a diagram describing a search request message generationalgorithm G according to the database search system of FIGS. 1 and 2;

FIG. 5 is a diagram describing a search response message generationalgorithm R according to the database search system of FIGS. 1 and 2;

FIG. 6 is a diagram describing a search result determination algorithm Daccording to the database search system of FIGS. 1 and 2;

FIG. 7 is a schematic diagram showing an exemplary hardwareconfiguration of a computer suitable for implementing the databaseserver and the client terminal according to the database search systemof FIGS. 1 and 2;

FIG. 8 is a diagram illustrating a prior art database search methodusing a one-way hash function; and

FIG. 9 is a diagram illustrating a prior art conventional databasesearch method using an OPE protocol.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 portrays an exemplary overall environment in which a system andmethod for searching a database or data sharing system for existence ofdata (alternatively referenced as the database searching system)according to the present invention may be used. The database searchingsystem includes a software programming code or computer program productthat is typically embedded within, or installed on a computer such as ahost server. Alternatively, the database searching system can be savedon a suitable storage medium such as a diskette, a CD, a hard drive, orlike devices. The database search system in this embodiment comprisesdatabase servers 10 and a client terminal 20.

The database servers 10 are servers that provide a search service ofdata stored in databases through a network.

The client terminal 20 is a terminal device used by a client of theservice provided by the database servers 10. The client terminal 20issues a search request to the database servers 10 for inquiring whetherthe database servers 10 have desired data.

In this embodiment, the database servers 10 are implemented asworkstations, personal computers, or other computers with networkcapability. The client terminal 20 is implemented as a workstation, apersonal computer, a PDA (Personal Digital Assistant), or otherinformation processing devices with network capability. The databaseservers 10 and the client terminal 20 exchange messages through anetwork (not shown). The network communication line may be either wiredor wireless.

As shown, this embodiment involves N database servers 10 to which theclient terminal 20 broadcasts the search request. Consequently, eachdatabase server 10 is referred to with an index 1 to N as needed, suchas the database servers 101, . . . , 10N. The database servers 101, . .. , 10N may also be collectively referenced as the database server 10 i(1≦i≦N).

FIG. 7 is a schematic diagram of an exemplary hardware configuration ofa computer suitable for implementing the database server 10 i and theclient terminal 20 according to this embodiment.

The computer comprises a CPU (Central Processing Unit) 701 as operationmeans; main memory 703 connected to the CPU 701 via a M/B (motherboard)chipset 702 and a CPU bus; a video card 704 also connected to the CPU701 via the M/B chipset 702 and an AGP (Accelerated Graphics Port); ahard disk 705, a network interface 706, and a USB port 707 connected tothe M/B chipset 702 via a PCI (Peripheral Component Interconnect) bus;and a floppy disk drive 709 and a keyboard/mouse 710 connected to theM/B chipset 702 via the PCI bus over a bridge circuit 708 and alow-speed bus such as an ISA (Industry Standard Architecture) bus.

FIG. 7 is a mere illustration of a hardware configuration of a computerfor realizing this embodiment; various other configurations to whichthis embodiment can be applied may also be employed. For example, onlyvideo memory may be provided instead of the video card 704, in whichcase the CPU 701 processes image data. Further, a CD-ROM (Compact DiscRead Only Memory) driver or a DVD-ROM (Digital Versatile Disc Read OnlyMemory) driver may be provided via an interface such as ATA (ATAttachment).

FIG. 2 shows a configuration of the database server 10 i and the clientterminal 20 in this embodiment.

The client terminal 20 comprises a search request message generationunit 21 for generating a search request message req from a data numberα. The data number α is a search item (a number applied to data owned bythe database server 10). The search request message generation unit 21uses a search request message generation algorithm G to be describedbelow.

The client terminal 20 also comprises a search result determination unit22 for outputting a value of 1 or 0 as a search result based on a searchresponse message ans_(i) from the database server 10 i using a searchresult determination algorithm D, also described below. The searchrequest message generation unit 21 and the search result determinationunit 22 are virtual software blocks provided by a program-controlled CPUin the information processing device that embodies the client terminal20.

Although not shown, the client terminal 20 includes a transceiver meansprovided by the program-controlled CPU and by a network interface. Theprogram that controls the CPU may be stored in and distributed as amagnetic disk, optical disk, semiconductor memory, or other storagemedia, or distributed through a network.

The database server 10 i comprises a database 11 and a search responsemessage generation unit 12. The search response message generation unit12 generates the search response message ans_(i) based on the searchrequest message, req, and a list of data stored in the database 11 usinga search response message generation algorithm R, which will bedescribed below.

The database 11 is provided in a storage device (such as a magnetic diskor an optical disk) of the computer that embodies the database server10. The search response message generation unit 12 is a virtual softwareblock provided by a program-controlled CPU in the computer. Although notshown, the database server 10 i includes a transceiver means provided bythe program-controlled CPU and by a network interface. The program thatcontrols the CPU may be stored in and distributed as a magnetic disk,optical disk, semiconductor memory, or other storage media, ordistributed through a network.

A list search (database search) according to this embodiment isformulated as follows.

A set of the database servers 10 i (called a server set hereafter) isrepresented as S={101, . . . , 10N}. Each of the database servers 101, .. . , 10N has n data items in the database. A unique data number isapplied to each data item. A set of all data numbers is represented asL. A list of data numbers of n data items stored in the database 11 isrepresented as L_(i)={x_(i), 0, . . . , x_(i), n-1}.

The database searching system comprises a search request messagegeneration algorithm, G(α), a search response message generationalgorithm, R(req, L_(i)), and a search result determination algorithm,D(ans_(i)). The search request message generation algorithm, G(α),receives an input of the data number α(∈ L)and outputs the searchrequest message req. The search response message generation algorithm,R(req, L_(i)), receives inputs of the search request message req and thelist L_(i) stored in the database 11, outputting the search responsemessage ans_(i). The search result determination algorithm, D(ans_(i))receives an input of the search response message ans_(i) and outputs asearch result {0, 1}. Where req=G(α) and ans_(i)=R(req, L_(i)), 1 isoutput if α∈ L_(i); otherwise, 0 is output.

The “list search” addressed by this embodiment of the database searchingsystem is formulated by generating and transmitting the search requestmessage, generating and transmitting the search response message, anddetermining the search result. The client terminal 20 generates thesearch request message, selecting the data number α∈ L of desired datatargeted by the search. The client terminal 20 generates the searchrequest message req=G(α) for searching for α using the search requestmessage generation algorithm G and sends the search request message reqto the entire server set S.

The database server 10 i generates and transmits the search responsemessage. Each of the database servers 101, . . . , 10N receives thesearch request message req and performs the following processing. Thedatabase server 10 i generates the search response messageans_(i)=R(req, L_(i)) from the search request message req and the listL_(i) using the search response message generation algorithm R, andsends the search response message ans_(i) to the client terminal 20.

The client terminal 20 determines the search result. The client terminal20 receives the search response message ans_(i) and obtainsres_(i)=D(ans_(i)) using the search result determination algorithm D.

This embodiment of the database searching system provides the searchrequest message generation algorithm G, the search response messagegeneration algorithm R, and the search result determination algorithm Dof the “list search” that meets the following conditions:

any database server 10 i can obtain no information on the data number αof desired data targeted by the search from the search request messagereq=G(α);

the client terminal 20 can obtain no information from the searchresponse message ans_(i) sent by the database server 10 i as a responseto the search request message req=G(α) on whether α∈ L_(i) holds; and

the size of the search request message req is independent of the numberN of the database servers 10 i that receive the search request.

The “list search” that meets these characteristics is called a “secretlist search with the secret search request.”

The first condition above prevents the database server 10 i fromknowing, from the search request message, information on which datanumber is targeted by for the search by the client terminal 20. Thiscondition is necessary for protecting the privacy of the client terminal20. The second condition above prevents the client terminal 20 fromobtaining excess information on the list in the database server 10 i.This condition is necessary for the secrecy of the database in thedatabase server 10 i. The third condition above improves efficiency.

The nature of the list search requires that the client terminal 20 querymany database servers 10 i on the network to locate a database server 10i having a data item that matches the data number that the clientterminal 20 desires. When the size of the search request message isindependent of the number of the database servers 10 i, the searchrequest message can be sent to numerous database servers 10 i.Therefore, the third condition above is a characteristic stronglydesired in view of service scalability.

This embodiment of the database searching system satisfies allconditions described above and provides the “secret list search with thesecret search request”. The following parameters are used by thedatabase searching system in this embodiment: large prime numbers p andq that meet q|p-1; and an element g of order q in a finite field Z_(p).The size of p and q is set such that the discrete logarithm problem fora group GF (q) formed with g as a generator is difficult in terms ofcomputational complexity.

Further, all operations are performed in mod p unless otherwise stated.Besides the formation in a prime field of order p, the formationcontemplated in the database searching system is also defined for anygroup GF (q) such that the discrete logarithm problem is difficult interms of computational complexity.

For example, the formation may be a group formed by lettingmultiplication operations in a prime field of order p correspond toaddition operations on a curve such as an elliptic curve over a certainfinite field. Alternatively, the formation may be a group formed byreplacing the prime number p with a number of powers of a prime numberp′, and replacing remainder operations modulo the prime number p′ withoperations in an extension field of GF (p′).

FIG. 3 describes the concept of the list search according to thisembodiment. In this embodiment of the database searching system, thislist search is performed by creating an nth-order polynomial f_(Si)(x)and using OPE to compute T_(i). The database server 10 i creates annth-order polynomial f_(Si)(x) having its data numbers x_(i, 0), . . . ,x_(i, n-1) as roots (that is, f_(Si)(x_(i, j)) where j=0, . . . , n-1).

To determine whether any database server 10 i has α∈ L_(i)), the clientterminal 20 causes each database server 10 i in the server set S to useOPE and compute T_(i), as follows:

T _(i) =g ^(tfsi(a)).

where t is a non-zero and secret random number randomly generated by thedatabase server 10 i for each response. Then, α is as follows:

T _(i) =g ^(tf) ^(Si) ^((α))

T_(i)=1 then α∈ L_(i)

T_(i)=1 then α∈ L_(i).

In the list search of this embodiment, when the client terminal 20 is toreceive g^(tfSi(α)) from the database server 10 i, the client terminal20 wants to hide α from the database server 10 i (to protect the privacyof the search request). On the other hand, the database server 10 iwants to hide g^(tfSi(α′)) (α′≠α) from the client terminal 20 (toprevent providing excess information on the list). With thisconsideration, an evaluation problem suitable for a special form of OPEbased on the DDH problem can be set.

Because of the nature of OPE, no information on α leaks to the databaseserver 10 i. Further, if α∈L_(i), then T_(i)=1 irrespective of the valueof t. Otherwise, T_(i) will be a non-1 and random value with nocorrelation with each search request, because t is randomly chosen foreach response. Therefore, the protocol for the list search according tothis embodiment causes no leakage of information on the polynomial forany number of search requests executed (the number being expressed as apolynomial in the size of q, that is, infinite in practice).

The construction of the search request message generation algorithm G,the search response message generation algorithm R, and the searchresult determination algorithm D is described below.

In the initial state, the client terminal 20 exposes public keys <g,e=g^(d)> corresponding to a private key d∈ Z_(q), and the server set Sexposes the number n of the data items in the list.

FIG. 4 describes the search request message generation algorithm G. Thealgorithm G is executed in the search request message generation unit 21of the client terminal 20. The algorithm G receives an input of thedesired search target data number α and outputs the search requestmessage req. In particular, it performs the following processing where

j=1, . . . , n.

Choose random r_(j)(∈ Z_(q)).

Compute (v_(j), u_(j)) with the public keys g and e:

v_(j)=g^(rj); and

u _(j) =e ^(rj) ·g ^(aj).

The search request message generation unit 21 outputs 2 n values

(v₁, u₁), . . . , (v_(n), u_(n)) computed in this manner as req. Theoutput search request message req is sent to the n database servers 10 ivia a network interface provided in the client terminal 20 as atransmission means and over the network.

FIG. 5 describes the search response message generation algorithm R. Thealgorithm R is executed in the search response message generation unit12 of the database server 10 i. The algorithm R receives an input of thesearch request message req and the list L_(i) stored in the database 11,and outputs the search response message ans_(i). In particular, itperforms the following processing by OPE.

Obtain the following nth-order polynomial

f _(S) _(i) (x)=Σ_(j=0) ^(n) a _(j) x ^(j) mod q

by expanding:

${f_{s_{i}}(X)} = {\prod\limits_{\gamma \in L_{i}}\; {\left( {x - \gamma} \right){mod}\mspace{14mu} q}}$

Choose random t, s(∈ Z_(q)).

Compute c₁ and c₂, as follows:

$c_{1} = {g^{s} \times {\prod\limits_{j = 1}^{n}\; \left( v_{j} \right)^{{ta}_{j}}}}$$c_{2} = {e^{s} \times g^{{ta}_{0}}{\prod\limits_{j = 1}^{n}\; \left( u_{j} \right)^{{ta}_{j}}}}$

The two values (c₁, c₂) computed in this manner are an evaluation resultof OPE, and the search response message generation unit 12 outputs thetwo values as ans_(i). The output search response message ans_(i) isreturned to the client terminal 20, which is the sender of the searchrequest message req, via a network interface provided in the databaseserver 10 i as transmission means and over the network.

FIG. 6 describes the search result determination algorithm D. Thealgorithm is executed in the search result determination unit 22 of theclient terminal 20. The algorithm D receives an input of the searchresponse message ans_(i) and outputs the search result res_(i). Inparticular, it performs the following processing:

Obtain T with the private key d, as follows:

$T = \frac{c_{2}}{c_{1}^{d}}$

If T=1, then output 1 as res_(i). If T≠1, then output 0 as res_(i).

From the output search result res_(i), the client using the clientterminal 20 can know which database server 10 i has the desired data.Then, the client may make another request for the desired data from thatdatabase server 10 i having the desired data.

Database systems to which this embodiment can be applied might be askill database, a distributed database, a treatise database, or apeer-to-peer database, for example.

A database of personal skills can be used to share information amongpeople and to communicate between persons. This may be called anelectronic-information version of business card exchange. Here, forsmooth database operation, providing excess information is undesirablebecause a personal skill is private information on the person. For ahuman resource database of a temporary agency, a person with aparticular skill may be extracted from the database. Then, providinginformation on all skills would reveal important secrets of the agency,which would be a problem.

In this example, the skills stored in the database may be arranged in alist with their data numbers, and a management server for the databasemay be the database server 10 i of this embodiment. A terminal devicecorresponding to the client terminal 20 may issue a search request whilehiding the data number of a desired skill. The database server 10 i thencannot know which skill (data item) the client terminal 20 has targetedby the search. The client terminal 20 also cannot know what kinds ofskills the database server 10 i stores as its data besides the requestedskill. Consequently, database management with protected privacy ispossible.

In recent years, home electric appliances with connection to an externalnetwork such as the Internet have been increasing. Among others, an ideathat is becoming practical is recording a broadcast program into a harddisk and sharing recorded data with others over the network to create adistributed database. Here, a client who wants to obtain recorded datahas to search for the recorded data stored in the hard disk connected tothe network. However, the client wants to keep secret the programtargeted by the search because it is private information. It is furtherdesired to hide a list of recorded data stored in the hard disk forprivacy reasons.

In this case, a unique data number may be applied to each recorded dataitem stored in the hard disk, and the host computer of the hard disk maybe the database server 10 i of the database search system. A terminaldevice corresponding to the client terminal 20 may issue a searchrequest while hiding the data number of the desired recorded data. Thedatabase server 10 i then cannot know what kind of recorded data (dataitem) the client terminal 20 has targeted with the search request. Theclient terminal 20 also cannot know what kinds of recorded data thedatabase server 10 i stores other than the requested recorded data.Consequently, database management with protected privacy is possible.

In addition, some databases of this kind charge for data searches. Forproper charging, the amount of a search result obtained in one search islimited. According to this embodiment, the client can obtain noinformation on data other than the data for which the search request isissued. Consequently, proper management of such paid systems ispossible.

As computerized submission of treatises is increasing, many websites onthe Internet provide lists of treatises belonging to the websites.However, due to copyright restrictions on the treatises, it may beundesirable to post treatise data itself in an open database. On theother hand, providing a treatise from a person to a person is oftenpossible. Therefore, it is desired to hide a list of distributabletreatises. On the other hand, a person searching the treatises will havea breach of privacy if the kind of treatise the person is targeting withtheir search is widely known.

In this case, the treatises stored in the database may be arranged in alist with their data numbers (treatise numbers), and a management serverfor the database may be the database server 10 i of this embodiment ofthe database search system. A terminal device corresponding to theclient terminal 20 may issue a search request while hiding the datanumber of a desired treatise.

The database server 10 i then cannot know what kind of treatise (dataitem) the client terminal 20 is targeting with the search request. Theclient terminal 20 also cannot know what kinds of treatises the databaseserver 10 i stores as its data besides the requested treatise.Consequently, the treatise may be provided to the searching person whilethe database server 10 i hides the treatise list and the searchingperson.

In recent years, there has been an explosive spread of use of databasesemploying peer-to-peer (P2P) network techniques. In a peer-to-peernetwork, each of an unspecific number of peers having different datashares the data with other peers and constructs a huge database as awhole. In searching this database, the database search according to thisembodiment may be used to find out where desired data is located.

Specifically, a peer that searches the database may be the clientterminal 20, and the other peers on the network may be the databaseservers 10 i of this embodiment. The peer corresponding to the clientterminal 20 broadcasts to the other peers a search request in which adesired data is kept secret. According to the method of this embodiment,a common search request may be used irrespective of the number of thedatabase servers 10 i (the size of the search request is independent ofthe number of the database servers 10 i). This is advantageous for adatabase search of this kind.

With this embodiment, it is possible to hide the data item involved withthe search request, as well as the list of the data items in thedatabase to be searched. This may contribute to a sound development of adatabase utilizing a peer-to-peer network.

As described above, the database search system can ensure the privacy ofa search request and the security of a database with high processingefficiency.

It is to be understood that the specific embodiments of the inventionthat have been described are merely illustrative of certain applicationof the principle of the present invention. Numerous modifications may bemade to the system and method for searching a database or data sharingsystem for existence of data invention described herein withoutdeparting from the spirit and scope of the present invention.

What is claimed is:
 1. A method for searching a database server having alist of data items stored therein, the method comprising: receiving asearch request message that includes an encryption of a desired dataitem from a client by the database server, wherein the desired data itemis kept secret from the database server; generating a search responsemessage by the database server, wherein the search response message isgenerated based on the search request message and based on the list ofdata items; and transmitting the search response message to the client,wherein whether the desired data item is stored in the database serveris determined based on the search response message, wherein the presenceof any other data items of the list of data items is not revealed to theclient.
 2. A method for determining by a client whether a desired dataitem is stored in a database server having a list of data items storedtherein, the method comprising: generating a search request message thatincludes an encryption of a desired data item by the client;transmitting the search request message to the database server, whereinthe desired data item is kept secret from the database server; receivingfrom the database server a search response message, wherein the searchresponse messaged is generated based on the search request message andbased on the list of data items; and determining whether the desireddata item is stored in the database server based on the search responsemessage, wherein the presence of any other data items of the list ofdata items is not revealed to the client.
 3. The method of claim 1,wherein generating the search response message further comprisesgenerating an n^(th)-order polynomial equation having the data items inthe list of data items as its roots and evaluating the n^(th)-orderpolynomial equation using the desired data item.
 4. The method of claim1, further comprising calculating a pair of values using a public key.5. The method of claim 1, wherein the desired data item is kept secretby encrypting the desired data item using a public key.
 6. The method ofclaim 1, further comprising determining by the client whether thedesired data item is stored in the database server using the searchresponse message and a private key.
 7. The method of claim 2, furthercomprising generating the search request message in accordance with apublic key.
 8. The method of claim 2, wherein keeping secret the desireddata item comprises encryption of a data number associated with thedesired data item.
 9. The method of claim 2, further comprisingdetermining whether the desired data item is stored in the databaseserver using a private key.
 10. The method of claim 2, furthercomprising transmitting the search request message to a plurality ofservers.
 11. A system for searching a database server having a list ofdata items stored therein, the system comprising: a search requestmessage generation unit that generates a search request message thatincludes an encryption of a desired data item transmitted from a clientto the database server, wherein the desired data item is kept secretfrom the database server; a search response message generation unit thatgenerates a search response message based on the search request messageand based on the list of data items, wherein the search response messageis used to determine whether the desired data item exists in thedatabase server without revealing the presence of any other data itemsof the list of data items to the client, and wherein the search responsemessage is transmitted from the database server to the client; and asearch result determination unit that determines whether the desireddata item is stored in the database server based on the search responsemessage.
 12. A system for determining by a client whether a desired dataitem is stored in a database server having a list of data items, thesystem comprising: a search request message generation unit thatgenerates a search request message that includes an encryption of adesired data item transmitted to the database server, wherein thedesired data item is kept secret from the database server; a searchresponse message generation unit that generates a search responsemessage based on the search request message and based on the list ofdata items, wherein the search response message is used to determinewhether the desired data item exists in the database server withoutrevealing the presence of any other data items of the list of data itemsto the client; and a search result determination unit that determineswhether the desired data item is stored in the database server based onthe search response message.
 13. The system of claim 11, wherein thesearch response message further comprises an n^(th)-order polynomialequation having the data items in the list of data items as its rootsand the n^(th)-order polynomial equation is evaluated using the desireddata item.
 14. The system of claim 11, further comprising a pair ofvalues calculated using a public key.
 15. The system of claim 11,wherein the desired data item is kept secret by encrypting the desireddata item using a public key.
 16. The system of claim 11, furthercomprising a determination by the client whether the desired data itemis stored in the database server using the search response message and aprivate key.
 17. The system of claim 12, wherein the search requestmessage is generated in accordance with a public key.
 18. The system ofclaim 12, wherein the desired data item is kept secret using anencryption of a data number associated with the desired data item. 19.The system of claim 12, further comprising a determination whether thedesired data item is stored in the database server using a private key.20. The system of claim 12, further comprising a transmission of thesearch request message to a plurality of servers.